The present invention relates to a video image printer for reproducing an image on a photosensitive medium based on a video signal, and more particularly to a video image printer for driving a liquid crystal light valve based on image information borne by a video signal, to allow light to be transmitted through the light valve for exposure of a photosensitive medium to reproduce an image thereon.
There has been a growing demand for video image printers which produce hard copies bearing reproduced images that have been displayed on monitor displays. Such video image printers are used in various applications. For example, a video image printer may be used in video shops or the like which provide a quick printout service. Medical organizations may find video image printouts useful as a diagnostic aid or for data storage. Video image printers may find home use for obtaining photoprints of desired displayed video images. It is desired that printouts or hard copies thus produced be of high image quality.
One known recording method employed by video image printers is a thermal recording process in which an image is printed on a heat-sensitive recording medium directly by a thermal head. However, the thermal recording process is disadvantageous in that the heat-sensitive recording medium is easily degraded and hence cannot be preserved over a long period of time, and it can record images in only two colors, i.e., black and white.
The above problems can be solved by a thermal image transfer recording process. According to this process, a recording medium is in the form of a plain sheet of paper, and a type ribbon composed of strips of magenta, cyan, and yellow is positioned adjacent to the recording medium. The colors that are carried by the type ribbon are thermally transferred to the recording medium by a thermal head. The thermal image transfer recording process is however employed in reproducing only a colored image of line art because the process is unable to reproduce continuous tone images due to difficulty in controlling the amount of coloring materials to be transferred.
Continuous tone images can be reproduced by a thermal sublimation process in which a sublimable ink is vaporized and transferred to a recording medium, in a quantity depending on the intensity of heat generated by a thermal head. The temperature of the thermal head can be varied by an electric current supplied to the thermal head, thus controlling the amount of ink to be vaporized. The process lends itself to the conversion of image densities into a multiplicity of tones. The thermal sublimation process requires that the recording medium be even and uniform on its surface. Therefore, the process usually employs a dedicated recording medium in the form of a sheet of paper coated with a polyester, and hence requires the expenditure of a high running cost.
Any of the recording methods referred to above is a thermal recording process, and needs to incorporate a heat dissipation device in recording equipment. The equipment which operates according to these methods is not small and light, and also not portable.
Another recording process known in the art is a laser beam scanning process. However, laser beam scanning recording devices are large because of the scanning of a recording medium with a laser beam. The laser beam scanning recording devices are also expensive since the light deflector for deflecting the laser beam is expensive.
Still another recording arrangement employs a piezoelectric device for atomizing and ejecting a liquid ink through nozzles. The recording system has a relatively low recording speed since it uses four nozzles for printing an image in a serial fashion. It is a big disadvantage of this recording system that these ink ejecting nozzles are often plugged. It is highly difficult to keep the nozzles in good operating conditions.